diff --git a/gates/pool.py b/gates/pool.py
index 1020bd2..a5865a1 100644
--- a/gates/pool.py
+++ b/gates/pool.py
@@ -44,6 +44,23 @@ def write_manifest(base_dir, pool_id, manifest):
     return manifest
 
 
+def next_image_name(manifest):
+    return f"img_{manifest.get('next_seq', 1):04d}.png"
+
+
+def add_image(base_dir, pool_id, data, ts=0):
+    m = read_manifest(base_dir, pool_id)
+    name = next_image_name(m)
+    d = pool_dir(base_dir, pool_id)
+    d.mkdir(parents=True, exist_ok=True)
+    with open(d / name, "wb") as f:
+        f.write(data)
+    m["slots"].append({"image": name, "mask": None, "label": "", "added": ts})
+    m["next_seq"] = m.get("next_seq", 1) + 1
+    write_manifest(base_dir, pool_id, m)
+    return m
+
+
 def rebuild_manifest(base_dir, pool_id):
     # Temporary stub — replaced in Task 7.
     return empty_manifest()
diff --git a/tests/test_pool.py b/tests/test_pool.py
index 44fa6e0..8666d2c 100644
--- a/tests/test_pool.py
+++ b/tests/test_pool.py
@@ -26,3 +26,26 @@ def test_write_is_atomic_no_partial_temp_left(tmp_path):
     pool.write_manifest(str(tmp_path), "p1", pool.empty_manifest())
     leftovers = list((tmp_path / "p1").glob("*.tmp"))
     assert leftovers == []
+
+
+def test_next_image_name_uses_next_seq():
+    m = pool.empty_manifest()
+    assert pool.next_image_name(m) == "img_0001.png"
+    m["next_seq"] = 42
+    assert pool.next_image_name(m) == "img_0042.png"
+
+
+def test_add_image_writes_file_and_appends_slot(tmp_path):
+    data = b"\x89PNG\r\n\x1a\n" + b"fake"  # bytes are written verbatim
+    m = pool.add_image(str(tmp_path), "p1", data, ts=123)
+    assert len(m["slots"]) == 1
+    slot = m["slots"][0]
+    assert slot == {"image": "img_0001.png", "mask": None, "label": "", "added": 123}
+    assert m["next_seq"] == 2
+    assert (tmp_path / "p1" / "img_0001.png").read_bytes() == data
+
+
+def test_add_image_monotonic_after_growth(tmp_path):
+    pool.add_image(str(tmp_path), "p1", b"a", ts=1)
+    m = pool.add_image(str(tmp_path), "p1", b"b", ts=2)
+    assert [s["image"] for s in m["slots"]] == ["img_0001.png", "img_0002.png"]